For example: Let's say you have a tedious selection you want to lasso, like the leg on a drawing you're doing, so that you can transform it. So you lasso it. But your hand twitched and you selected too much of the other pieces of the drawing!

In most programs that feature select tools, it's okay to be sloppy on the initial select, as you can deselect portions of the selected area using a "subtract" select, which lets you iterate more precise carvings for usability.  "Add" does a similar-but-opposite action, growing the selection where you drag to cover parts you missed.

These features are completely missing in Krita, heavily hampering useability for people who use transformations / move often in their sketches. If you select incorrectly, you have to do it over and over again until it is -just- right, which is near-impossible with the lack of anti-aliasing on select tools.

Reproducible: Always

Steps to Reproduce:
1. Use any shape of selection tool.
2. Attempt to find way to refine your selection after making it.
3. Realize that there is no way to do this.

Actual Results:  
The path-select tool is good, but many people don't like dealing with path tools / find them slow. So there's really not a way to do this.

Expected Results:  
Using shortcuts like holding Alt (subtract) or Ctrl (add), it should be possible to add / subtract to your existing selection without erasing it and starting an entirely new selection.